By MARK McCAIN

Published: August 20, 1989

DAYS are numbered for the jumble of stores, restaurants and other commercial establishments in Grand Central Terminal. As leases expire, tenants are moving out or accepting month-to-month extensions from the Metropolitan Transportation Authority, which is completing plans for a retail complex more akin to a chic urban mall.

''The atmosphere will be upgraded and uplifted,'' said Jeanne Giordano, a deputy director of real estate for the New York State authority. ''Right now, the terminal is a little confusing. So we're designing better ways for people to circulate through the terminal - both to reach their transportation needs and to find whatever shops or restaurants they might want to visit.''

Changes over the next four years will be vast. The elegant waiting room just off 42d Street, now a campsite for homeless people, will become a retail center. The Kodak Colorama sign in the central concourse will be dismantled, and the east gallery it has blocked since 1950 probably will accommodate an expensive restaurant. Below the sign, a visitors' center is expected to replace a Chemical Bank branch that opened in 1978.

Pizzerias and ice cream stands that flank walkways in the terminal will have to comply with new design and quality standards or be replaced by more fashionable tenants.

''But we're not going to just lease space to the highest bidders - we are much more understanding of what helps make a project like this succeed,'' Ms. Giordano said. ''For instance, right now in Grand Central we really don't have any healthy and good take-out food to heat up at home - even though that has become very popular all over Manhattan. We also don't have a really good gift shop or a place that sells maps and other information about New York.''

The retail revamping is part of a master plan for the second 75 years of Grand Central Terminal, which was completed in 1913 and boasts one of the most majestic interiors in New York City. But after World War II it fell into disrepair as Americans deserted trains for cars and airplanes.

''We want to make the terminal grand again, which is something I think everyone would welcome,'' said Peter Stangl, president of Metro-North Commuter Railroad Company, an M.T.A. division that manages the terminal.

''First and foremost, we want Grand Central to be a terrific train station,'' he said. ''Secondly, it's important to restore the building's architectural integrity. And thirdly, we want to improve the way we use space for retail purposes. It's been a little helter-skelter in recent years.''

BUT the state authority faces significant challenges in its efforts to uplift Grand Central's retail sector, which now spans 100,000 square feet and will cover about 25 percent more space when renovations are completed.

About 500,000 people pass through the terminal each day. But some have no time for shopping; others have no money for it.

Unlike daily commuters, who shave travel time down to the second, train passengers bound for distant cities tend to arrive early, with spare time for shopping. At its peak in the 1920's, Grand Central served as the point of departure or final destination for more than 500 long-distance trains a day; now only 18 a day use it and by 1990 Amtrak will be shifting them to Pennsylvania Station, on 34th Street. After that, only commuter trains will leave from Grand Central.

While commuters rush through the terminal, homeless people linger there with blankets and plastic lawn chairs. Panhandlers work the ticket lines, and garbage bags full of belongings are stashed beside a photo exhibit celebrating ''A New Life for Grand Central.''

''Grand Central is a wonderful space, but it has some very real problems,'' said Bradley C. Mendelson, managing director of retail leasing for Edward S. Gordon Company, a Manhattan real estate concern. ''At Grand Central, everyone is always late to the office or late to a train, which doesn't promote a mindset for shopping. And I don't think anybody believes the homeless problem will lend itself to a quick solution.''

Nonetheless, a few upscale establishments already are prospering at Grand Central. Every day droves of people head down to the dining room of the Oyster Bar, which has been a fixture in the terminal's lower level from day one.

Lines of customers often spill out of Zaro's Bread Basket, a bakery and delicatessen that took empty space beneath the terminal's west balcony 12 years ago and created an instant hit. A few years later, the Cafe at Grand Central opened on the balcony above, and most days it easily fills its 100 seats for lunch. Across the concourse stands the Kodak sign that yields about $40,000 a month in rent.

''I'll be sad to see that sign go because it's my scenery,'' said John McFadden, owner-manager of the cafe. ''However, I guess all the restoration work in the terminal makes sense, just so long as a new generation of M.T.A. management doesn't decide to change it back to a more commercial look five years from now.''

Like other tenants, Mr. McFadden feels nervous about his future because the M.T.A. has not revealed who will be invited to remain. ''I'm hoping this cafe will exist as long as Grand Central continues to stand,'' he remarked, ''but I have no idea what the M.T.A. is thinking.''

Andrew Zaro of Zaro's Bread Basket says that since 1986 he has wanted to spend $750,000 on renovations, but has been unable to obtain any information from the transportation authority about its intentions for the space the bakery now occupies.

''Things have changed since we built this store in 1977, and we would love to have the opportunity to reinvest money so we could sharpen our act up,'' said Mr. Zaro, who still has several years remaining on his lease. ''But the M.T.A. keeps us in the dark.''

Earlier this year Chemical Bank sent notices to customers announcing that its Grand Central branch would close in May. But the M.T.A. agreed to grant a one-year lease extension, for about $20,000 a month.

''WE'RE really not trying to be ogres about this,'' said Ms. Giordano, referring to the authority's relations with commercial tenants. ''We're not trying to breed terror.''

Part of the authority's problem is that although changes have been in the works for several years, approval by its board of governors has yet to come.

''Any time you do something like this, you create uncertainty for tenants,'' Mr. Stangl said. ''There are a lot of tenants in the terminal we'd like to work with, but we can't make long-term commitments right now.''

Early this fall the authority's governing board is expected to vote on the new framework for retailing strategy in Grand Central. The finishing touches of that framework are now being completed by Williams Jackson Ewing, a Baltimore concern that developed a vast retail mall in Washington's Union Station, which reopened last year. Meanwhile, a lot of retail space is sitting vacant in Grand Central. Along its 42d Street frontage, three of seven storefronts are empty. And inside, an occasional art exhibit helps fill empty display windows. About 80 commercial tenants are renting space today in Grand Central, including a private tennis club that operates two courts above the main waiting room in space CBS once used for television studios. On the main level the tenants are more democratic: Ticketron, Fotomat, Dunkin Donuts, Mr. Potato, Mrs. Fields.

''The M.T.A. has to create an environment that goes beyond just coffee cups and cash machines,'' said Faith Consolo, vice president of Garrick-Aug Associates Store Leasing, a Manhattan brokerage. ''Right now, the terminal is not well thought out, and it seems that M.T.A. gets so caught up in making the right decision that no decisions get made.

''What the terminal needs,'' she said, ''is a better mix of services and affordable but attractive shops. Nobody's going to buy a $500 suitcase at Grand Central, but they would probably buy a $19 nylon tote.''

Kodak sign in main terminal will be dismantled and Chemical Bank branch beneath it may be replaced with a visitors' center; Zaro's Bread Basket, beneath west balcony (The New York Times/Neal Boenzi)